Monday 15 February 2021

An Amphbian's Missed Chance

Plenty of PT-76 tanks were in service in the early 2000s. The Vietnamese army alone had about 500 units and the total number of functional but obsolete tanks was about 1000. An idea came up to modernize these tanks and give them as second life.

The PT-76 was simple and reliable, but also quite obsolete. The 76 mm D-56TS gun was no longer satisfactory for modern combat. A modern light tank needs to be able to fight personnel, unarmoured and lightly armoured vehicles, anti-tank weapons, helicopters, and low flying airplanes. Because of this, the primary goal in modernizing the PT-76 was improving its firepower. At this time TsNII Burevestnik in Nizhniy Novgorod offered the AU-220M artillery system, which had its own long history.

The PT-76-57 during gunnery trials at the Gorohovets proving grounds.

In 1967 KB-3 of the Gorkiy Machinebuilding Factory began working on a single 57 mm autocannon system for ships. This system was indexed A-220. By 1978 the system passed state trials. Despite successful results, it was not accepted into service, primarily due to the low effectiveness of 57 mm ammunition. Almost 25 years later TsNII Burevestnik (reorganized from the KB-3 in 1970) decided to modernize the system and offer it to the Ministry of Defense as well as foreign buyers.

The PT-76-57 during gunnery trials at the Gorohovets proving grounds.

The oscillating part of the A-220M was nearly unchanged, but the tracking mechanism, gun controls, and fire control system were completely changed. The system weighed 6 tons, which limited its applications. The lightened AU-220M variant was developed.

Different variants were planned depending on the capacity of the platform, and so the mass ranged from 2.5 to 4.5 tons. The combat module designed for the PT-76 tank fit 92 rounds in a mechanized ammunition rack. It weighed 3.6 tons, which meant it could easily be installed in a PT-76 instead of the stock turret.

As an aside, it was not just Russian developers worked on modernizing the PT-76. Among other, the Israeli company Nimda modernized a few dozen PT-76 tanks belonging to Indonesian marines in the late 90s. These vehicles were indexed PT-76/2000 or jus PT-2000. 

A Cockerill Mk.III A-2 90 mm Belgian gun and a new set of sights including a laser rangefinder were installed in the stock turret. The coaxial PKT machine gun was replaced with the Belgian FN GPMG. The V-6V engine was replaced with a 290 hp American Detroit Diesel 6V92T, which increased the tank's top speed to 58 kph. The PT-76/2000 is still in service to this day.

The PT-76-57 during gunnery trials at the Gorohovets proving grounds.

During the development of the PT-76 with a 57 mm gun, TsNII Burevestnik compared its effectiveness with the stock D-56TS variant as well as the one armed with the Cockerill Mk.III A-2. The muzzle velocity of the 76 mm shell was 820 m/s, the Belgian gun had 700 m/s, and the prospective 57 mm gun had a muzzle velocity of 1000 m/s. 

The rate of fire of the PT-76 with a D-56TS gun was 7-8 RPM, the Cockerill had 5-7 TPM. The 57 mm autocannon offered a rate of fire of up to 120 RPM in single shot, short bursts, or continuous fire of up to 18 rounds in a row. The PT-76 carried 40 shells for the 76 mm gun, only 12 shells for the Belgian gun, or 92 rounds of 57 mm ammunition.

The 57 mm gun and its magazine.

The Belgian gun had superior penetration: 185 mm at 500 meters or 100 mm at 2000 meters. However, this was not enough to fight new tanks, and more than enough to fight lightly armoured targets. The 57 mm gun had similar penetration to the D-56TS at 2000 meters (75 mm) and surpassed it at 500 meters (115 mm vs 98). The ability to fire bursts increased the odds of defeating an anti-tank rocket launcher at 2.5 km with the first shot (burst) to 60% compared to 20% with the D-56TS and 30% with the Cockerill. The odds of taking out an APC from 2 km were 70% compared to 30 and 40% respectively.

In trials the 57 mm gun in the AU-220M system defeated 100 mm of homogeneous armour from 1120 meters. Unlike its predecessors, the gun had a 60% chance of hitting a helicopter from 4 km. The single shot cannons proved incapable of fighting helicopters at any range.

Turret of the BM-57 module.

The BM-57 turret was welded together from rolled armour thick enough to protect from 7.62 mm bullets. Several different turret shapes were designed. In addition to protecting the tank from fire, the turret was meant to protect it from detection by radar, and thus designs with different angles of armour were tested, including inverse sloping. However, this increased complexity and cost, and so a rather simple shape was used in the end.

The turret housed a commander/gunner and a loader. The autocannon was based on the 57 mm S-60 AA gun and used the same ammunition: HE-T 53-UOR-281-U and AP-T 53-UBR-281U. The gun was equipped with an automatic loading system with a system for extracting spent casings.

The PT-76-57 at TsNII Burevestnik during inspection after gunnery trials.

The mechanized ready rack containing 20 rounds is located to the left of the gun and allows switching ammunition types in 2-4 seconds. The ready rack is refilled by the loader during breaks from firing from the 72 round main rack. To reduce concentration of gun fumes in the fighting compartment, the system is covered with a protective cowl and equipped with a gas ejection system. The tank also carries 2000 rounds of ammunition for the coaxial PKTM machine gun.

A modern fire control system is designed for day or night observation of ground, water, or airborne targets, rangefinding, and firing either from standstill, on the move, or from water with the chosen type of ammunition and desired length of burst. The fire control system consists of a sight/observation system, fire correction computer, and two plane stabilizer. 

BM-57 combat module.

The sight and observation system includes a Liga-S independently stabilized night and day time sight and backup 1P67 optical sight used in case of power loss. Liga-S has three visual channels, a night vision channel, and a laser rangefinder. The sight has a range of 800-1200 meters at night (in passive/active modes). The rangefinder can measure ranges from 500 m to 6000 m. There are three magnification modes: 1x, 5x, and 14x.

The 1P67 AA sight allows spotting of moving and stationary targets, aiming the gun at land, water, and airborne targets, and firing with either the cannon or machine gun. It has two magnification modes: 1.2x and 4x. The commander can use his remote control to select different fire modes and types of ammunition.

Liga-S and 1P67 sights.

The fire correction computer and stabilizer improve effectiveness of fire against stationary and moving targets. The system includes a 1V539M ballistic computer, barrel wear sensor, ammunition thermometer, weather sensor, and movement sensors (yaw, heading, speed). The ballistic computer reads data from all sensors and calculates the lead to effectively strike a moving target. Thanks to a dual plane stabilizer that can move the gun at a speed of up to 20 degrees per second (or up to 35 degrees during traverse) firing on the move or from water has the same precision as firing in place.

The commander and loader have TNPT-1 (one each) and TNPO-170 (two each) observation devices.

Commander/gunner's controls. The Liga-S sight can be seen to the left, the ballistic computer is in the center, the control panel and ammunition counter is to the bottom right.

The control panels allow the commander to control ammunition racks and tell the commander and loader the amount of remaining ammunition. The automated ammunition rack allows the module to eschew the use of a dual feed system, such as the one used by the BMP-2. The loader can refill the automated ready rack from the mechanized rack when the gun is not firing. It takes no more than 6 seconds to load one round.

The issue of mobility was tackled after the firepower was dealt with. A 300 hp UTD-20 engine used on the BMP-2 was installed instead of a 240 hp V-6V engine. The cooling system was improved with better radiators. The gearbox was replaced along with the engine, the main and final drive clutches were modernized. The power to weight ratio increased to 20 hp/ton, which allowed the top speed on paved roads to rise from 44 to 60 kph and on dirt roads from 32 to 42 kph.

Commander/gunner's station from above. The eyepiece for the 1P67 backup sight can be seen to the left. The Liga-S sight is in the center. The ballistic computer is to the right.

The vehicle was unofficially named PT-76-57, and a prototype was built in 2002. The tank was sent to the Gorohovets proving grounds for trials to evaluate the usefulness of this modernization. The following range data was obtained based on practical fire from standstill, on the move, and from water.
  • Against medium tanks and MBTs (penetration of side armour or damage without penetration): up to 1000 meters.
  • Against APCs, IFVs, and SPGs: up to 2100 meters.
  • Against towed artillery: up to 2300 meters.
  • Against unarmoured vehicles: up to 2500 meters.
  • Against personnel in fortifications: up to 1300 meters.
  • Against personnel in the open: up to 3500 meters.
  • Against helicopters: up to 4000 meters.
The conclusions stated that the PT-76-57 can deal with the full range of issues faced by a light tank as a part of a mobile force. The firepower was 5.5 times higher than a PT-76B tank, mobility increased by 35%, reliability improved by 1.35 times. The overall combat effectiveness was 2.7 times higher.

Designs of the BM-57 turret.

Since the combat module was developed to modernize existing PT-76 tanks, the issue of simplicity and cost was a pressing one. The modernization consisted of two main parts: replacement of the fighting compartment and replacement of the transmission and engine group. No serious changes were made to the hull. The modernization would coincide with refurbishment, which would reset the vehicle's warranty period. An agreement was even reached with the Volgograd Tractor Factory to restart production of spare parts for the PT-76B.

However, despite all of the advantages of the design none of the nations that uses the PT-76 showed any interest in the project. Only one experimental BM-57 module has been built so far.

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